This invention relates to bag making machines and, particularly, to machines for cutting and sealing a plastic web or tube to an intended bag configuration. Although a number of machines of this type are in use in the manufacture of such bags, they are far from ideal and have a number of significant limitations. By way of example, a typical bag making machine has an arrangement at one end for supporting the axle of a roll of sheet material from which the web is unwound. The web is drawn to a cutting and sealing station by a pair of nip rollers.
The rollers are rotated to advance a fixed length of web to a cutting and sealing station where the web is then cut and sealed to form the bag. The rollers typically are driven by a mechanical crank arrangement which tends to seriously limit the speed of the machine. Also among the difficulties encountered with the prior devices is that the drive rolls, which often are of the order of four or five feet long, sometimes display a tendency to twist or flutter as they rotate, particularly when an effort is made to operate the machine at its highest speed range. This causes variations in the grip which the rollers apply to the web which results in lateral shifting and misalignment of the web. This difficulty can be particularly acute when one side of the bag is gussetted and in which the gussetted region is effectively four layers thick while the other regions of the web are only two layers thick. In order to overcome the flutter or twist tendency, the rolls, which are supported at their ends, typically have been made of a heavy and massive construction. The use of such massive rolls, however, further restricts the speed of the machine because of the high inertia of the roll which makes it difficult to start and stop rapidly. Also, the use of such massive rolls generally requires a heavy duty type of drive mechanism for the rollers.
Another difficulty often encountered with the prior art devices relates to the heat sealing of the thermoplastic bags. Typically, the heat sealing is effected by applying heated wires to the bag material to briefly melt a region of the thermoplastic web and fuse it together. Often, the heat sealing operation is also employed to cut through the web to sever it, as is the case when making "side weld" bags. It has been common practice to heat the wire more than is necessary to melt, fuse and cut through the thermoplastic sheet. Not infrequently, this results in a tendency for the sealed region of the bag to remain tacky for a longer period than is desirable and in some instances, the bag may tend to stick to the wire. This interferes with the operation of the machine and often require that the machine be shut down for maintenance. In addition, it should be noted that when making side weld bags, a cut and sealed bag is withdrawn from the machine by pulling it from the cutting and sealing station. Usually, the bag is tensioned slightly while it is being cut and sealed which causes the melted thermoplastic to be drawn out at the cut and sealed region which results in a thinner and weaker bag construction at that seam. Also, the overheating of the wire reduces the useful life of the wire.
A further difficulty encountered with prior bag making machines relates to the cutting and sealing of bags bearing printed material. The webs of material employed in the making of such bags are printed at regularly spaced intervals along the web and it is important that the cutting and sealing of the web occurs at the precise, desired location between repetitive printings. This, in turn, requires that the drive mechanism be controlled to feed the web in precise increments to insure proper registration of the printed material on the finished bag. Such accuracy sometimes is complicated considerably if the web has been stretched. Often, the machine must be shut down for readjustment when printed bags are being made. In general, the prior machines include a mechanical crank-type drive which is set to advance the web slightly more than the intended bag length. A photoelectric sensing device is incorporated into the machine to sense the advancement of an imprinted registration mark on the web and the output from the photocell is employed to actuate a brake to stop the drive mechanism. Such machines are operated at as high as production rate as is possible and the brakes tend to heat up and do not always stop the advancement of the bag precisely in registry with the cutting and sealing station. The error thus introduced becomes cumulative which necessitates readjustment of the machine at frequent intervals.
It is among the general objects of the invention to provide a bag making machine which avoids the above and other difficulties.